The fibrous protein collagen accounts for 20 to 25 per cent of the total protein of mammals, where it plays supportive roles in skins, bones, tendons, and other fabrics. The X-ray diffraction method (at "wide-angles") has provided triple-chain coiled-coil models for the arrangement of polypeptides in collagen macromolecules and (at "small angles") has discovered and measured the periodicity with which matter is distributed along native fibrils. Supplementary information has been accumulating through notable biochemical, physio-chemical and electron- optical investigations during recent years. This present research program has a principal objective of re-applying small-angle X-ray diffraction in the light of the new developments. The immediate objectives of this investigation are the determination of the ways in which collagen molecules aggregate to form the native fibril. To this end, models containing band locations and molecular overlap and hole regions have been used in conjunction with small-angle intensity data, to devise and refine electron density distributions along the fibrillar axes. Since more than two-thirds of the amino-acid sequences of the collagen molecule are now known, it has become feasible also to explore how the side chains along molecular surfaces influence the aggregation. Comparative X-ray observations on typical strategic collagens, such as rat-tail and kangaroo-tail tendons, teleostean fish mandibular tendons, and shark-fin elastoidin are also being undertaken.